The proposed research is designed to assess the relationship between the detailed molecular structure of a phospholipid bilayer and its ability a) to bind prothrombin and b) to enhance the rate of conversion of prothrombin to thrombin by Factor Xa in the presence of an optimal concentration of calcium ions, in vitro (functional definition of thrombogenicity). As a working hypothesis, it is proposed that the combined effects of prothrombin and Ca2 ion may induce in fluid lipid bilayers the particular lipid rearrangements (lateral domain formation) necessary to promote the binding of prothrombin to the membrane, resulting in its subsequent activation. In order to test this hypothesis, the following approaches will be used: 1) preparation of synthetic phospholipid bilayer vesicles as models for the lipid portion of biological membranes involved in coagulation; 2) determination of the physical state of phospholipid bilayers thru the construction of phase diagrams, by the techniques of fluorescence depolarization spectroscopy and differential scanning calorimetry; 3) characterization of the binding of prothrombin fragment 1, as a model for prothrombin binding to phospholipid vesicles; 4) measurement of phospholipid bilayer thrombogenicity; 5) determination of the combined effect of prothrombin fragment 1 and Ca2 ion on the physical state of the phospholipid vesicles; 6) characterization of those phospholipid bilayer properties (e.g., head group composition, cholesterol content and surface curvature) that affect fragment 1 binding and bilayer surface thrombogenicity. The long-term goal of this research is the continued development of a practical model system on which further studies of protein-lipid interactions in blood coagulation may continue.